We develop a computationally efficient approach to compute the waveforms and the dispersion curve... more We develop a computationally efficient approach to compute the waveforms and the dispersion curves for fault-zone trapped waves guided by arbitrary transversely isotropic across-fault velocity models. The approach is based on a Green's function type representation for F L and F R type fault-zone trapped waves. The model can be used for simulation of the waveforms generated by both infinite line sources (2-D) and point sources (3-D). The numerical scheme is based on a high order finite element approximation and, to increase computational efficiency, we make use of absorbing boundary conditions and mass lumping of finite element matrices.
The focus of this article is a process whereby lower crustal crystalline and schistose rocks can ... more The focus of this article is a process whereby lower crustal crystalline and schistose rocks can rise to the surface, with the Tehachapi Mountains in California being the case in point. As a prime example of the lower crust, these mountains expose Cretaceous gneisses that formed 25-30 km down in the Sierra Nevada batholith and appear to be underlain by the ensimatic Rand schist. Integrated geophysical and geological studies by the CALCRUST program have produced a cross section through this post-Mid-Cretaceous structure and suggest a general model for its development. Seismic reflection and refraction profiles show that the batholithic rocks dip northward as a tilted slab and extend beneath the southern end of the San Joaquin Basin's Tejon embayment. Two south dipping reverse faults on the rim of the Tejon embayment were discovered in the reflection data and verified in the field. The faults have a combined separation of several kilometers and cut through an upper crustal reflection zone that projects to the surface outcrop of the Rand schist. The upper and lower crusts are separated by a zone of laterally discontinuous reflectors. Reflections from the lower crust form a wedge, the base of which is a nearly flat Moho at 33 km. Regional geological relations and gravity models both suggest that the reflective zone corresponds to the Rand schist and the newly recognized faults account for its Neogene exposure. Alternatively, the reflective zone maybe part of the gneiss complex, suggesting that the schist either lies deeper or is not present under the gneisses. If the Rand schist underlies the Tehachapi Mountains and Mojave region to their south, a model for their evolution can be constructed from regional geological relations. It seems that during Late Cretaceous Laramide subduction the protolith of the schist was thrust eastward beneath the Mojave. Along this portion of the Cordilleran batholithic belt the subduction was evidently at very low angles. The bottom of the batholith was removed and replaced by a thick section of schist, fluids from which weakened the overlying batholith. This thickened crust collapsed by horizontal flow in the schist and faulting of the upper crust into flat-lying slabs. When emplacement of the schist ended in latest Cretaceous/earliest Paleocene, the underlying mantle rose, compensating for the extension and providing material for magmatic underplating. In the Neogene, transpression and rotation of the upper crust along the San Andreas and Garlock faults resulted in the exposure of the schist. 19254 to CALCRUST, EAR87-08266 and 89-04063 to J. Saleeby, and EAR91-19263 and 91-19263 to P. Malin. J. Plescia of the Jet Propulsion Laboratory, Pasadena, California, provided gravity data and several early models from which Figure 3 was developed. We are grateful for his open and kind help. The authors have benefited from conversations with J. Crowell, T. McEvilly, S. Richard, and J. Sharry and many others. We would like to thank ARCO for providing their "vibrator buster" test equipment to check the vibrators before the survey and the Tejon Ranch Company for access to their property in the Tehachapi Mountains. We thank E. Karageorgi for generating the filtered field data at the Earth Science Division, Lawrence Berkeley Laboratory. Comments on the
The local fault and dike structures in Puna, southeastern Hawaii, are of interest both in terms o... more The local fault and dike structures in Puna, southeastern Hawaii, are of interest both in terms of electricity production and volcanic hazard monitoring. The geothermal power plant at Puna has a 30 MW capacity and is built on a section of the Kilauea Lower East Rift Zone that was resurfaced by lava flows as recently as 1955 and 1960. The Puna Borehole Network was established in 2006 in order to provide detailed seismic data about the Puna geothermal field. The array consists of eight 3-component borehole seismometers. The instrument depths range from 24 to 210 m (80 to 690 ft); five stations with 2 Hz geophones and three with 4.5 Hz geophones. The large majority of events were < M0.5 and occurred at depths between 2 and 3 km. The size, location, and depth of the microearthquakes suggest that power plant activity affects local seismicity. Earthquake depths increase from NE to SW, trending up the rift zone towards the Kilauea summit. Depths also increase from the study area to the SE, consistent with active normal faulting along Kilauea's south flank. Shear wave polarization indicates that the active, fluid-filled fracture system trends SW-NE, consistent with the orientation of the LERZ. Double difference relocation suggests an intersecting network of fractures with both NE and approximately NW trends. 3-D tomographic analyses of P-wave velocity, S-wave velocity, and the Vp/Vs ratio are also presented. An area of anomalously fast P-wave velocity at the relatively shallow depth of 2.5 km may be evidence for a dense, gabbroic intrusion. This intrusion may underlie and be the parent of a body of dacitic melt that was discovered under the geothermal field during drilling.
Journal Of Geophysical Research: Solid Earth, Sep 1, 2022
Geothermal energy is considered an important and growing source of low-carbon-footprint energy. D... more Geothermal energy is considered an important and growing source of low-carbon-footprint energy. Development of deep Enhanced Geothermal Systems (EGS) using massive fluid injection (hydraulic fracturing) to improve reservoir permeability often leads to the occurrence of induced seismicity (e.g., . Large earthquakes associated with anthropogenic fluid injection activities such as in Basel, Switzerland (e.g.,
Earthquakes are frequently accompanied by public reports of audible low-frequency noises. In 2018... more Earthquakes are frequently accompanied by public reports of audible low-frequency noises. In 2018, public reports of booms or thunder-like noises were linked to induced earthquakes during an Engineered Geothermal System project in the Helsinki Metropolitan area. In response, two microphone arrays were deployed to record and study these acoustic signals while stimulation at the drill site continued. During the 11 day deployment, we find 39 earthquakes accompanied by possible atmospheric acoustic signals. Moment magnitudes of these events ranged from $$-0.07$$ - 0.07 to 1.87 with located depths of 4.8–6.5 km. Analysis of the largest event revealed a broadband frequency content, including in the audible range, and high apparent velocities across the arrays. We conclude that the audible noises were generated by local ground reverberation during the arrival of seismic body waves. The inclusion of acoustic monitoring at future geothermal development projects will be beneficial for studyin...
A seismic network was installed in Helsinki, Finland to monitor the response to an ∼6-kilometer-d... more A seismic network was installed in Helsinki, Finland to monitor the response to an ∼6-kilometer-deep geothermal stimulation experiment in 2018. We present initial results of multiple induced earthquake seismogram and ambient wavefield analyses. The used data are from parts of the borehole network deployed by the operating St1 Deep Heat Company, from surface broadband sensors and 100 geophones installed by the Institute of Seismology, University of Helsinki, and from Finnish National Seismic Network stations. Records collected in the urban environment contain many signals associated with anthropogenic activity. This results in time- and frequency-dependent variations of the signal-to-noise ratio of earthquake records from a 260-meter-deep borehole sensor compared to the combined signals of 24 collocated surface array sensors. Manual relocations of ∼500 events indicate three distinct zones of induced earthquake activity that are consistent with the three clusters of seismicity identif...
Near–real-time seismic monitoring and injection control limits induced earthquakes in urban geoth... more Near–real-time seismic monitoring and injection control limits induced earthquakes in urban geothermal reservoir stimulation.
The Marmara section of the North Anatolian Fault Zone (NAFZ) runs under water and is located less... more The Marmara section of the North Anatolian Fault Zone (NAFZ) runs under water and is located less than 20 km from the 15-million-person population center of Istanbul in its eastern portion. Based on historical seismicity data, recurrence times forecast an impending magnitude M>7 earthquake for this region. The permanent GONAF (Geophysical Observatory at the North Anatolian Fault) has been installed around this section to help capture the seismic and strain activity preceding, during, and after such an anticipated event.
Synthetic fault-zone trapped wave (FZTW) dispersion curves and amplitude responses for F L (Love)... more Synthetic fault-zone trapped wave (FZTW) dispersion curves and amplitude responses for F L (Love) and F R (Rayleigh) type phases are analysed in transversely isotropic 1-D elastic models. We explore the effects of velocity gradients, anisotropy, source location and mechanism. These experiments suggest: (i) A smooth exponentially decaying velocity model produces a significantly different dispersion curve to that of a three-layer model, with the main difference being that Airy phases are not produced. (ii) The FZTW dispersion and amplitude information of a waveguide with transverse-isotropy depends mostly on the Shear wave velocities in the direction parallel with the fault, particularly if the fault zone to country-rock velocity contrast is small. In this low velocity contrast situation, fully isotropic approximations to a transversely isotropic velocity model can be made. (iii) Fault-aligned fractures and/or bedding in the fault zone that cause transverse-isotropy enhance the amplitude and wave-train length of the F R type FZTW. (iv) Moving the source and/or receiver away from the fault zone removes the higher frequencies first, similar to attenuation. (v) In most physically realistic cases, the radial component of the F R type FZTW is significantly smaller in amplitude than the transverse.
Discriminating between a creeping and a locked status of active faults is of central relevance to... more Discriminating between a creeping and a locked status of active faults is of central relevance to characterize potential rupture scenarios of future earthquakes and the associated seismic hazard for nearby population centres. In this respect, highly similar earthquakes that repeatedly activate the same patch of an active fault portion are an important diagnostic tool to identify and possibly even quantify the amount of fault creep. Here, we present a refined hypocentre catalogue for the Marmara region in northwestern Turkey, where a magnitude M up to 7.4 earthquake is expected in the near future. Based on waveform cross-correlation for selected spatial seismicity clusters, we identify two magnitude M ∼ 2.8 repeater pairs. These repeaters were identified as being indicative of fault creep based on the selection criteria applied to the waveforms. They are located below the western part of the Marmara section of the North Anatolian Fault Zone and are the largest reported repeaters for the larger Marmara region. While the eastern portion of the Marmara seismic gap has been identified to be locked, only sparse information on the deformation status has been reported for its western part. Our findings indicate that the western Marmara section deforms aseismically to a substantial extent, which reduces the probability for this region to host a nucleation point for the pending Marmara earthquake. This is of relevance, since a nucleation of the Marmara event in the west and subsequent eastward rupture propagation towards the Istanbul metropolitan region would result in a substantially higher seismic hazard and resulting risk than if the earthquake would nucleate in the east and thus propagate westward away from the population centre Istanbul.
Fault Zone Guided Waves (FZGWs) have been observed for the first time within New Zealand's tr... more Fault Zone Guided Waves (FZGWs) have been observed for the first time within New Zealand's transpressional continental plate boundary, the Alpine Fault, which is late in its typical seismic cycle. Ongoing study of these phases provides the opportunity to monitor interseismic conditions in the fault zone. Distinctive dispersive seismic codas (~7–35 Hz) have been recorded on shallow borehole seismometers installed within 20 m of the principal slip zone. Near the central Alpine Fault, known for low background seismicity, FZGW‐generating microseismic events are located beyond the catchment‐scale partitioning of the fault indicating lateral connectivity of the low‐velocity zone immediately below the near‐surface segmentation. Initial modeling of the low‐velocity zone indicates a waveguide width of 60–200 m with a 10–40% reduction in S wave velocity, similar to that inferred for the fault core of other mature plate boundary faults such as the San Andreas and North Anatolian Faults.
Bulletin of the Seismological Society of America, 2016
Using the first dataset available from the downhole Geophysical Observatory of the North Anatolia... more Using the first dataset available from the downhole Geophysical Observatory of the North Anatolian Fault, we investigated near-surface seismic-wave propagation on the Tuzla Peninsula, Istanbul, Turkey. We selected a dataset of 26 seismograms recorded at Tuzla at sensor depths of 0, 71, 144, 215, and 288 m. To determine near-surface velocities and attenuation structures, the waveforms from all sensors were pairwise deconvolved and stacked. This produced low-noise empirical Green's functions for each borehole depth interval. From the Green's functions, we identified reflections from the free surface and a low-velocity layer between ∼90 and ∼140 m depth. The presence of a low-velocity zone was also confirmed by a sonic log run in the borehole. This structure, plus high near-surface P-and S-wave velocities of ∼3600-4100 and ∼1800 m=s, lead to complex interference effects between upgoing and downgoing waves. As a result, the determination of quality factors (Q) with standard spectral ratio techniques was not possible. Instead, we forward modeled the Green's functions in the time domain to determine effective Q values and to refine our velocity estimates. The effective Q P values for the depth intervals of 0-71, 0-144, 0-215, and 0-288 m were found to be 19, 35, 39, and 42, respectively. For the S waves, we obtained an effective Q S of 20 in the depth interval of 0-288 m. Considering the assumptions made in our modeling approach, it is evident that these effective quality factors are biased by impedance contrasts between our observation points. Our results show that, even after correcting for a free-surface factor of 2, the motion at the surface was found to be 1.7 times greater than that at 71 m depth. Our efforts also illustrate some of the difficulties of dealing with site effects in a strongly heterogeneous subsurface. Online Material: Plots of resistivity and caliper logs and the spectra of all 26 events.
In this Final Report we present and discuss the materials that we are using as a basis for a publ... more In this Final Report we present and discuss the materials that we are using as a basis for a publication submitted to the Bulletin of the Seismological Society of America on the results of our project. These material are presented in the subsequent pages.
Permanently cemented sensors have proven to be longterm stable with non-deteriorating coupling an... more Permanently cemented sensors have proven to be longterm stable with non-deteriorating coupling and borehole integrity. However, each type needs to be carefully selected and planned according to the research aims. A convenient case study is provided by a new installation of downhole seismometers along the shoreline of the eastern Marmara Sea in Turkey. These stations are being integrated into the regional net for monitoring the North Anatolian Fault Zone. Here we discuss its design, installation, and first results. We conclude that, despite the logistical challenges and installation costs, the superior quality of downhole data puts this technique at the forefront of applied and fundamental research. Downhole monitoring and intelligent well completions have initially been employed in offshore oil and gas wells. The International Ocean Drilling Program (IODP) has optimized science-driven subsea well completions by installing sensors for permanent operation in mid-ocean boreholes (Davis and Becker 2007). All of these downhole geophysical observatory installations required, as a minimum, a deployment system, the measurement sensors, a sensoranchoring system, and a data recording system. Permanent Downhole Monitoring (PDM) installations on land have different challenges. In their current state of the art, there are two principle PDM designs available in industry and academia, both of which are mature enough to provide safe and reliable long-term monitoring operation Abstract Downhole sensors of different types and in various environments provide substantial benefit to signal quality. They also add the depth dimension to measurements performed at the Earths' surface. Sensor types that particularly benefit from downhole installation due to the absence of near-surface noise include piezometers, seismometers, strainmeters, thermometers, and tiltmeters. Likewise, geochemical and environmental measurements in a borehole help eliminate near-surface weathering and cultural effects. Installations from a few hundred meter deep to a few kilometer deep dramatically reduce surface noise levels-the latter noticeably also reduces the hypocentral distance for shallow microearthquakes. The laying out of a borehole network is always a compromise of local boundary conditions and the involved drilling costs. The installation depth and procedure for a long-term downhole observatory can range from time limited installations, with a retrieval option, to permanently cemented sensors.
Roughly 7-10 geothermal wells are drilled in order to provide 1 well producing enough steam to dr... more Roughly 7-10 geothermal wells are drilled in order to provide 1 well producing enough steam to drive a turbine. This well-drilling sunk cost is, however, far from unique to geothermal reservoirs. Rather it is endemic to crustal reservoirs of all fluid types: convention and non-conventional hydrocarbon, groundwater, fossil flow mineralisation, as well as geothermal. E.g., representative of the vast statistical data for onshore oil field well productivity, in the year 2009 the state of Texas had 60 wells producing an average of 645 barrels/day, 2984 wells producing an average of 88 barrels/day, and 80770 oil field wells producing an average of 1 barrel/day. As there is no intrinsic reason to consider geothermal fields significantly different from oil fields (except that the pay fluids are different), the multi-state multi-year US oil field well producer statistics are strong indicators of how rare it is to drill crustal reservoir 'producers': for purely blind drilling, for every geothermal producer, the intrinsic odds are for 50 non-producers (with an additional 1300 wells that would never be considered for drilling). At $100/barrel value of oil, oil field well sunk costs are manageable. At $1/barrel value of hot water, geothermal drilling sunk costs requires a fundamentally different well drilling strategy. Historically geothermal wells are drilled with reference to surface manifestations and geological interpretations of subsurface structure. Given what lognormal-distribution well productivity data tell us about the strong heterogeneity of reservoir flow systems, we can assert that we need greatly improved subsurface flow structure data to guide the drill bit to achieve more costeffective hydrothermal heat extraction. We can take the necessary step by looking below the reservoir surface via innovative use of standard multi-channel seismic sensor array technology as deployed over shale body reservoirs subjected to repeated massive hydrofracture treatments along horizontal wellbores. Unlike oil field flow systems, which are non-flowing until disturbed by reservoir operations, geothermal reservoir flow systems are naturally acoustically noisy, with the noise levels scaling in rough proportion to the volume of naturally occurring fluid flow. Surface sensor array data have demonstrated that acoustic noise generated by shale body fracking practice contain sufficient signal levels that specialised use of otherwise standard seismic processing technology can reliably locate a large range of reservoir subsurface flow structures, most importantly the large-scale reservoir flow patterns associated with large-scale but spatially elusive fracture channels endemic to crustal rock. While the dismal oil field well productivity statistics amply reflects the deficiency of drilling without valid information about subsurface flow targets, it is now possible to identify the fundamental physical reason behind poor well productivities, take technical steps to acquire necessary reservoir flow structure information, and actively upgrade drill site locations to guide the drill bit to the main flow structures in the reservoir fracture system.
SEG Technical Program Expanded Abstracts 2007, 2007
The San Andreas Fault Observatory at Depth provides the most comprehensive set of data on the str... more The San Andreas Fault Observatory at Depth provides the most comprehensive set of data on the structure and dynamics of the San Andreas fault. We use two independent experiments recorded by the seismometer arrays of the SAFOD Pilot and Main Holes to resolve the localized structure of the San Andreas fault zone and of an intermediate fault zone at depth. From Pilot Hole recordings of the drilling noise coming from the Main Hole, we reconstruct the waves that propagate between the Pilot Hole sensors and use them to image the fault zone structure. The use of correlated drilling noise leads to a high-resolution image of a major transform fault zone. Another independent image is generated from the detonation of a surface explosive charge recorded at a large 178-sensor array placed in the Main Hole. The images reveal the San Andreas fault as well as an active blind fault zone that could potentially rupture. This is confirmed by two independent methods. The structure and the activity of the imaged faults is of critical importance in understanding the current stress state and activity of the San Andreas fault system.
We have been able to map the size, depth, and fracture density of possible shallow geothermal ene... more We have been able to map the size, depth, and fracture density of possible shallow geothermal energy sources by analyzing microearthquake data in volcanically active areas. In this "volcanoseismic" approach to exploration and monitoring, we search for seismicity gaps, seismic velocity ratios, S-wave attenuation and splitting, reflected arrivals, and converted waves. Our analysis of shear wave split data for fracture density in the Mammoth field show two potentially fractured areas that are potential targets for drilling exploratory wells. Vp/Vs ratios in two Olkaria fields can be related to reservoir fluid phases. This ratio is 1.58 in the East Production Field and corresponds to a decrease in P-wave velocity in the area with low-pressure steam saturation in the reservoir. It is 1.71 in the high-pressure liquid-saturated North East Field. We have compared the ratios with down hole pressure, temperature and fluid chemical change measurements. These data show that Vp/Vs values depend on reservoir fluid phases and can be useful tools for reservoir monitoring.
Active and passive seismic experiments show that the southern Sierra, despite standing 1.8 to 2.8... more Active and passive seismic experiments show that the southern Sierra, despite standing 1.8 to 2.8 kilometers above its surroundings, is underlain by crust of similar seismic thickness, about 30 to 40 kilometers. Thermobarometry of xenolith suites and magne- totelluric profiles indicate that the upper mantle is eclogitic to depths of 60 kilometers beneath the western and central parts of the range, but little subcrustal lithosphere is present beneath the eastern High Sierra and adjacent Basin and Range. These and other data imply the crust of both the High Sierra and Basin and Range thinned by a factor of 2 since 20 million years ago, at odds with purported late Cenozoic regional uplift of some 2 kilometers. What holds up high mountain belts on continents? The Earth's two highest belts, the Himalaya-Tibet collision zone and the central Andes, are supported by Airy-type crustal roots 70 to 80 km thick, almost twice that of adjacent lowlands (1). A 30to 35km increase in crustal thickness should raise elevation by about 4500 m, in agreement with observed differences in elevation in these two cases (3). The Sierra Nevada, one of the major mountain ranges in North America, lies at an elevation of 2800 m but is enigmatic. It contains the highest point in the lower 48 states (Mount Whitney, 4419 m), yet just a short distance away, Death Valley lies below sea level, within a zone of strong crustal extension (4). In addition, conflicting seismic interpretations have been presented as to whether the High Sierra (roughly the eastern third of the range) is underlain by a crust 55 km thick (5) or by crust only 30 to 40 km thick (6), similar to that of surrounding lowlands of the Basin
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